A calcium sulfonate complex grease is excellent in heat resistance and water resistance and is thus often applied to a sliding part around an automobile engine, a bearing of a rolling machine or the like for iron and steel, and outdoor gear (see, for instance, Patent Literatures 1 and 2).
Such a calcium sulfonate complex grease is a metal soap grease made of a metal complex soap, which is excellent in lubricity but has a grease structure unlikely to be maintained for a long duration of time at a high temperature.
Accordingly, a grease composition containing a thickener containing organic bentonite and an auxiliary thickener component is suggested (see, for instance, Patent Literature 3). It is also disclosed that the auxiliary thickener component is a metal complex soap, polyurea, fluorocarbon resin, N-substituted terephthalamic acid metal salt or calcium sulfonate complex. According to Patent Literature 3, a mixture of the organic bentonite and the auxiliary thickener component is used as the thickener to provide a grease composition excellent in rust resistance, extreme pressure property, water resistance and lubrication life.